The present invention relates to a method in accordance with the preamble of claim 1.
Such a method is disclosed in the PCT application Wo 96/08050.
Various proposals exist in the prior art for producing, for example, a matrix plate in a fuel cell.
The oldest proposal comprises the so-called hot press technique. In a comparatively complicated manner this technique yields comparatively thick plates. The result of this, on the one hand, is an increase in production costs and, on the other hand, a reduction in the efficiency of the cell.
An alternative for producing matrix plates is a process which is similar to the papermaking process. This involves mixing pulp comprising a binder, water and the supporting materials of the matrix. The matrix material then adheres to the pulp, giving rise to floccules. Via the filtration process the floccules are separated from the water material, and the binder is removed by subsequent baking. This process is attractive since the suspension medium used is inexpensive, environment-friendly water. A drawback is its controllability and, in particular, the pore size and the pore size distribution. It did not prove possible to fabricate matrix plates reproducibly in this way on a commercially acceptable scale.
The third most commonly used technique is the so-called "tape casting".
This involves a suspension in which a binder, fibres and the matrix material are present being cast onto a flat table and the suspension medium present therein being evaporated. A green matrix plate is then produced which can either be sintered before being placed into a fuel cell, after which the electrolyte can be introduced, or is placed directly into the fuel cell. In the latter case the electrolyte is introduced in some other way.
When the suspension was being prepared it was not uncommon to employ vacuum conditions in order thus to regulate the viscosity of the fluid.
Such a method is described, for example, in the article "Review of Carbonate Fuel Cell Matrix and Electrolyte" by H. C. Maru et al. in Proceedings of 2nd Symposium on MCFC Technology, Vol. 90-16, pp. 121-136. The binder described therein was dissolved in a mixture of xylenes and ethanol. When the suspension is heated there is the problem, in particular, of removing xylene. It is no longer permissible for a solvent such as xylene to be discharged into the environment, and recycling leads to a considerable increase in costs.
A similar consideration applies to the use of acrylate as a binder which is dissolved in a mixture of acetone, dichloromethane and petroleum naphtha.
In the case of the method described in the abovementioned PCT application 96/08050 it was found that in the process of forming that suspension, solid particles will agglomerate, resulting in a decrease in the strength of the tape. This is caused by uneven dispersion of the additions and a pore structure which is not well defined. Moreover the introduction of additions such as fibre strengtheners, dissolution retardants and carbonate proved particularly difficult. Upon introduction of such substances agglomeration has likewise been observed, which counteracts the strength, on the one hand, and the dispersal of these substances, on the other hand.
Moreover it was found that this production gives rise to rejects and waste which cannot be put to any further use.